Engine with cylinder deactivation

ABSTRACT

A multi-cylinder spark ignition internal combustion engine is described having two banks of cylinders 10a, 10b. One bank of cylinders may be selectively disabled by cutting off its fuel supply while continuing to receive air. The exhaust system includes an NO x  trap 20 to store NO x  gases while the exhaust gases contain excess air. During part load operation, the engine is run with one bank of cylinders disabled most of the time during which NO x  gases are stored in the NO x  trap 20. In order to permit the trap 20 to be regenerated or purged periodically, both bank are fired at is the same time for short intervals to supply a stoichiometric or reducing mixture to the exhaust system.

FIELD OF THE INVENTION

The present invention relates to a multi-cylinder spark ignitioninternal combustion engine having two banks of cylinders supplied withintake air through a common main throttle, and disabling means forselectively deactivating one bank of cylinders by cutting off its fuelsupply.

BACKGROUND OF THE INVENTION

Systems for cylinder deactivation have been proposed previously, inorder to achieve improved fuel economy and reduced emission when theengine is operating at part load. Such systems rely on the fact thatspark ignition engines operate less efficiently at low load because ofthe pumping losses caused by throttling. Especially in a large engine,it is more efficient to run one bank of cylinder under higher load thantwo banks under lesser load, while producing the same output power.Cutting off the fuel supply to one bank of cylinders achieves thedesired reduction in fuel consumption but when the disabled cylindersare still allowed to pump air, this upsets the stoichiometry of theexhaust gases and interferes with the operation of the catalyticconverter. The presence of excess air in the exhaust gases means thatthe catalytic converter cannot neutralise NO_(x) present in the exhaustgases, as this requires a stoichiometric or reducing atmosphere. Forthis reason, known systems take special steps during cylinderdeactivation to avoid air reaching the catalytic converter through thedisabled cylinders. The steps that have been proposed for this purposeinclude maintaining the intake and exhaust valves of the disabledcylinders permanently shut, or running the disabled cylinders with 100%EGR. Both these proposals have disadvantages in that valve disablementis costly to implement and switching to 100% EGR gives rise to problemsin controlling the combustion during the periods of changeover betweennormal operation and deactivation. Also, undesirable leakage of EGRgases into the intake system of firing cylinders is difficult to avoid.

JP-A-55 029002 discloses an engine have two groups of cylinders 1-3 and4-6. Under high low, both groups of cylinders are operational but underlight load, cylinders 1-3 are disabled. The exhaust gases from bothgroups of cylinder pass through a main catalytic converter which ispreceded by a first oxygen sensor. The exhaust gases from cylinders 4-6additionally pass through another catalytic converter arranged upstreamof the common catalytic converter and itself preceded by a second oxygensensor. The first oxygen sensor sets the fuel quantity during high loadoperation and the second sets the fuel when only one group of cylinderis firing. During part load, the main catalytic converter and the firstoxygen sensor tend to cool down and the engine is forced to run on allcylinders for a short time to heat the main catalytic converter wheneverits temperature is sensed by a detector to be dropping below a minimumthreshold.

JP-A-55 49549 discloses an engine with two groups of cylinders that canbe selectively deactivated during part load operation. The engineexhaust system has three catalytic converters one main converter commonto both groups of cylinders and two further converters arranged upstreamof the main catalytic converter, each associated with a respective oneof the two groups of the cylinders. Each time that the engine isswitched from running on both groups of cylinders to only one group, thegroup of cylinders selected for deactivation is alternated. As a result,neither group of the cylinders is allowed to run cold and neither groupis subjected to wear at a different rate from the other group.

SUMMARY OF THE INVENTION

With a view to mitigating the foregoing disadvantages, the inventionprovides in accordance with a first aspect a multi-cylinder sparkignition internal combustion engine having two groups of cylinders, anddisabling means for selectively deactivating one group of cylinders bycutting off its fuel supply, wherein the two groups of cylinders areconnected to a common exhaust system containing a catalytic converter,the disabling means are operative to interrupt the fuel supply to onegroup of cylinders during part load operation so as to deactivate saidone group of cylinders while supplying air to said one group ofcylinders, and means are provided for resupplying fuel to said one groupof cylinders at periodic intervals to reactivate said one group ofcylinders, characterised in that the common exhaust system furthercomprises an NO_(x) trap and in that the duration of the intervals ofreactivation are sufficient to regenerate the NO_(x) trap.

During deactivation of one group of cylinders, air pumped through thosecylinders reaches the exhaust system to make the catalytic converteroperate only as an oxidation catalyst. Such NO_(x) as is produced duringthis time by the firing cylinders is stored in the NO_(x) trap. Atperiodic intervals, when both groups of cylinders are activatedsimultaneously, the exhaust mixture is returned to a stoichiometric orreducing mixture to neutralise the NO_(x) stored in the NO_(x) trap,thereby regenerating or purging the NO_(x) trap.

Another problem with disabling one group of cylinders is that if thecylinder disablement is prolonged, the group will risk a build up of oiland deposits within the cylinders.

According to a second aspect of the invention, there is provided amulti-cylinder spark ignition internal combustion engine having twogroups of cylinders, and disabling means for selectively deactivatingone group of cylinders by cutting off its fuel supply, wherein the twogroups of cylinders are connected to a common exhaust system containinga catalytic converter, and disabling means are operative during partload operation to interrupt the fuel supply so as to deactivate onegroup of cylinders at a time while supplying air to the disabled groupof cylinders, characterised in that the exhaust system includes anNO_(x) trap, the disabling means are operative to interrupt the fuelsupply alternately to the groups of cylinders during part loadoperation, and in that during changeover of the deactivation, there areintervals during which both groups of cylinders are activatedsimultaneously, the intervals having sufficient duration to regeneratethe NO_(x) trap.

In this aspect of the invention, the groups are alternately deactivatedso that the groups are subjected to equal wear and deposits that may beformed on the combustion chambers during cylinder deactivation will beburnt off more regularly and equally in both groups of cylinders.Preferably, the intake system has compensation means to reduce the airsupply to the two groups of cylinders during the interval when they areactivated simultaneously in order to avoid a sudden change in the engineoutput power.

The compensation means may comprise an electronic throttle that isregulated by a control system to maintain constant output power duringthe intervals when both groups of cylinders are activatedsimultaneously. Alternatively, the compensation means may comprise anON/OFF valve arranged in series with an auxiliary throttle in a passagebypassing the main throttle, the auxiliary and main throttles beingganged main throttle, the auxiliary and main throttles being ganged suchthat the flow through the two passages when the ON/OFF valve is open isalways in the same predetermined ratio to the mass air flow through themain throttle alone. In this case, the size of the auxiliary throttleand bypass passage may be calibrated such that the output power when themain throttle alone supplies air to the two banks of cylinders is thesame as the output power when both the main and auxiliary throttlessupply air to only one of the two banks. In this way, the complexity ofan electronic throttle can be avoided and replaced by a simple ON/OFFvalve in series with the auxiliary throttle.

While one bank of cylinders is deactivated, the other bank works underhigher load and produces NO_(x) gases in the exhaust system. These gasescannot be reduced in the three-way catalytic converter because thedisabled bank of cylinders continues to supply air and create anoxidising atmosphere. in the exhaust system. The three-way catalyticconverter therefore acts only as an oxidation catalyst to neutralise HCand CO in the exhaust gases and the NO_(x) trap, which itselfincorporated a three-way catalyst, is relied upon to store the NO_(x)gases until such time as they too can be neutralised when the NO_(x)trap is purged by supplying it with a stoichiometric or reducingatmosphere.

The NO_(x) trap has only a limited capacity but the invention allowsfreedom in setting the time between purging to avoid saturation of thetrap. The frequency with which the engine is operated with both banks ofcylinders activated simultaneously can be set as desired to ensure thatthe trap remains at a high storage efficiency. During these intervals,the fuelling can be set to achieve, as desired, a stoichiometric or areducing atmosphere in the exhaust gases passing through the catalyticconverter, to regenerate or purge the NO_(x) trap fully.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described further, by way of example, withreference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of an internal combustion engine,

FIGS. 2 and 3 are timing diagrams showing two alternative methods offuelling the internal combustion engine in FIG. 1,

FIG. 4 is a view similar to FIG. 1 of an embodiment having a modifiedintake system,

FIGS. 4a and 4b show a detail of the embodiment of FIG. 4 in alternativepositions of the valve supplying air to the intake manifolds of the twobanks of cylinders.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 an internal combustion engine has two banks of cylinders 10aand 10b having intake manifolds 12a and 12b and exhaust manifolds 14aand 14b, respectively. The exhaust manifolds 14a and 14b are joined toone another at a section 14 that precedes an after treatment systemconsisting of a catalytic converter 16, a burner chamber 18 having anigniter 22 and a NO_(x) trap 20, which itself contains a three-waycatalyst.

The intake system for both banks of cylinders comprises a mass air flowmeter 30 connected in series with a main throttle 32 that provides airto both intake manifolds 12a and 12b. In addition, the intake systemcomprises a bypass passage 38 containing a second throttle 34 gangedwith the main throttle 32 and an ON/OFF valve 36 for selectively openingand closing the bypass passage 38 depending on whether one or both banksof cylinders of the engine are activated.

Under high load operation, the ON/OFF valve 36 occupies the positionillustrated to disable the bypass passage 38. Fuel is metered to bothbanks of cylinders so that both banks fire normally and produce anexhaust gas mixture that is stoichiometric and can be purified by thethree-way catalysts.

During low and part load operation, the fuel supply to one of the banks10a and 10b is shut off while the other bank continues to fire normally.The air supply to the deactivated bank of cylinders is not discontinuedand these cylinders pass air into the exhaust system. The three-waycatalytic converter in the after treatment system can now only operateas an oxidation catalyst in this oxidising is atmosphere and the NO_(x)produced by the firing bank of cylinders cannot be neutralised. Toovercome this problem, the invention provides the NO_(x) trap to storethe NO_(x) gases and prevent them from being discharged to ambientatmosphere.

The NO_(x) trap has only a finite capacity and this mode of operationcannot be maintained indefinitely if NO_(x) gases are not to be releasedto the atmosphere. For this reason it is necessary to regenerate orpurge the NO_(x) trap at regular intervals by running the engine in sucha manner as to produce a reducing or stoichiometric exhaust mixture.This is achieved by periodically running both banks of cylinderssimultaneously by supplying fuel to both banks for a duration longenough to purge the NO_(x) trap.

The engine of FIG. 1 can be operated in one of two modes. In the firstmode the same bank of cylinders is always deactivated while in thesecond mode the deactivation alternates between the two banks ofcylinders. The first mode is represented by the fuel timing diagramshown in FIG. 2 in which fuel supply is permanently ON to the first bankof cylinders and pulsed ON at regular intervals to the second bank ofcylinders. The second mode on the other hand is represented by FIG. 3 inwhich both banks of cylinders are switched ON and OFF with the samemark-to-space ratio as one another, this mark-to-space ratio beingslightly in excess of 1:1 so that at the changeover between banks thereare defined brief purge intervals during which both banks of cylindersare activated simultaneously.

Both modes of operation of the engine achieve the desired purging of theNO_(x) trap but the second mode has the advantage that the banks aresubjected to equal wear and deposits are removed more regularly from thedisabled cylinders.

At the times that the engine operates with all cylinders firing, it willtend to produce more output power than when one bank is deactivated fora given position of the main throttle 32. The purpose of the ON/OFFvalve 36 is to avoid changes in engine output power during the purgeintervals and during the changeover between one bank and two banksoperation. When one bank is disabled, the ON/OFF valve 36 is turned toits fully open position to allow air to flow through the bypass passage38 and the second throttle 34. This latter throttle 34 is ganged tooperate in unison with the main throttle 32 and, for a given position ofthe main throttle 32, supplies the correct amount of compensation airflow such that the output power from the engine when one bank ofcylinders is deactivated is the same as the output power when both banksof cylinders are firing.

Exhaust gas ignition systems (EGI) have previously been proposed toaccelerate the light-off of a catalytic converter. The engine isintentionally run with an excessively rich mixture so that the exhaustgases contain hydrocarbons, carbon monoxide and hydrogen and additionalair is introduced directly into the exhaust system to produce anignitable mixture that is burnt immediately upstream of the catalyticconverter to bring it quickly to its light-off temperature during coldstarts. The burner chamber 18 is provided in the exhaust after treatmentsystem in FIG. 1 for this purpose but in the described embodiment ofthis invention, it is possible to avoid the need for an expensive sourceof additional air. If one bank of cylinders is run with a very richmixture and the other bank is deactivated but continues to receive air,then the resultant mixture will be ignitable in the burner chamber 18using the igniter 22. If the firing cylinders receive the fuel thatshould have been burnt by both banks of cylinders, they will be runningexcessively rich but the resultant exhaust gas mixture reaching theburner 18 will still be stoichiometric and burn completely. The heatreleased will quickly bring the NO_(x) trap which also contains athree-way catalyst to its light-off temperature.

The embodiment of FIG. 1 suffers from the disadvantage that the disabledbank of cylinders will still be partially throttled and would beperforming unnecessary pumping work against the manifold vacuum. Thisdisadvantage is avoided in the embodiment of FIG. 4 in that unthrottledambient air is supplied to the deactivated bank of cylinders in order toreduce the pumping loss to a minimum.

In the embodiment of FIG. 4 like numerals have been used to designatecomponents previously described by reference to FIG. 1 in order to avoidunnecessary repetition. The essential difference resides in theconnection between the main throttle 32 and the intake manifolds 12a and12b which in this case includes a diverter valve 40. For ease ofdescription the bypass passage 38 has been omitted it being assumed inthis case that the throttle 32 is an electronic throttle but a bypasspassage may be used as previously described if preferred to maintainconstant output power regardless of engine operating mode.

The diverter valve 40 has two inlet and two outlet ports. The firstinlet port, which has no reference numeral is connected to the throttle32 and the mass air flow meter 30. The second inlet port 46 is directlyconnected to ambient air and the two outlet ports 42 and 44 leadrespectively to the intake manifolds 12a and 12b. The valve has arotatable diverter element which is V-shaped in cross-section and can bemoved between the three positions shown in FIGS. 4, 4a and 4brespectively.

In the position shown in FIG. 4 the diverter element points at thethrottle 32 and obstructs the port 46 completely. Only air passing themass air flow meter 30 reaches the intake manifold 12a and 12b and thevalve 40 splits the air in equal amounts. This is the position occupiedby the valve 40 during normal operation with all cylinders firing.

The rotation of the diverter element to the position shown in FIG. 4ahas the effect of connecting the intake manifold 12a to the air passingthe intake throttle 32 and the mass air flow meter 30, while connectingthe intake manifold 12b to the ambient without throttling the air. Thisis the position adopted by the valve 40 when the second bank ofcylinders 10b is deactivated. The first bank of cylinders 10a nowoperates normally while the second bank of cylinders 10b operates withthe minimum pumping work and delivers air to the exhaust system.

If the same bank of cylinders is disabled every time, then the valve 40need only be capable of movement between the positions shown in FIGS. 4and 4a. If however it is desired to be able to switch the deactivationalternately between banks of cylinders, then the valve 40 can be movedfurther to the position shown in FIG. 4b. From the symmetry with FIG. 4ait will be appreciated that the only difference this will make is thatthe first bank of cylinders 10a would be disabled instead of the secondbank 10b.

An advantage of the embodiment of FIG. 4 is that it is very tolerant toleakage in the diverter valve 40. If any leakage does occur, air willenter the firing cylinders. This will not disturb the combustion processbut merely cause the mixture strength to be weakened slightly. If theengine is calibrated to supply a nominally stoichiometric mixture to thefiring bank of cylinders, based on the air flow measured by the mass airflow meter, any leakage that occurs will make the mixture slightlyleaner than stoichiometric, which is advantageous in ensuring lowhydrocarbon and carbon monoxide in the feed gases supplied to the aftertreatment system. NO_(x) may be increased in the feed gases but thestorage of the NO_(x) in a trap and the subsequent purging of the trapwill prevent this pollutant from being discharged to atmosphere. Thusthe aftertreatment system can be effective in controlling the dischargeof the three main noxious gases without the critical control of thestoichiometry of the exhaust gases that is required when using athree-way catalyst.

I claim:
 1. A multi-cylinder spark ignition internal combustion enginecomprising:an intake system having a main throttle; two banks ofcylinders connected to said intake system and a common exhaust system,with said exhaust system having a catalytic converter and a NO_(x) trap;disabling means for selectively deactivating one bank of cylinders byinterrupting fuel supply to said one bank of cylinders during part loadoperation while supplying air to said one bank of cylinders; and, meansfor resupplying fuel to said one bank of cylinders at periodic intervalsto reactivate said one bank of cylinders, with a duration of saidintervals being sufficient to regenerate said NO_(x) trap.
 2. An engineaccording to claim 1, wherein said intake system comprises acompensation means to reduce air supply to each said bank of cylinderswhen each said bank is activated simultaneously, thereby avoiding asudden change in engine output.
 3. An engine according to claim 2,wherein said compensation means comprises an electronically actuatedthrottle coupled to an electronic control system.
 4. An engine accordingto claim 2, wherein said compensation means comprises an ON/OFF valvearranged in series with a bypass throttle arranged in a bypass passagebypassing a main throttle arranged in a main passage, with said main andbypass throttles being ganged such that flow through said main andbypass passages when said ON/OFF valve is open is in a samepredetermined ratio to a mass air flow through said main throttle alone.5. An engine according to claim 1, wherein, when one bank of cylindersis deactivated, said one bank of cylinders communicates with ambient airthrough a diverter valve so that air supplied to said one bank ofcylinders is unthrottled.
 6. An engine according to claim 5, whereinsaid diverter valve comprises:two inlets connected to ambient air andsaid main throttle, respectively; two outlets, each connected to anintake manifold of a respective bank of cylinders; and, a rotatableelement having a first position, in which said inlet connected toambient air is obstructed while said inlet connected to said mainthrottle is connected simultaneously to both outlets, and a secondposition, in which said main throttle is connected to only one of saidoutlets while said inlet connected to ambient air is connected to theother of said outlets.
 7. A multi-cylinder spark ignition internalcombustion engine comprising:an intake system having a main throttle;two banks of cylinders connected to said intake system and a commonexhaust system, with said exhaust system having a catalytic converterand a NO_(x) trap; and, disabling means for selectively deactivating onebank of cylinders by alternately interrupting fuel supply to said twobanks of cylinders during part load operation, thereby deactivating onebank of cylinders at a time, while supplying air to said deactivatedbank of cylinders, therebeing intervals during changeover ofdeactivation in which both said banks of cylinders are activatedsimultaneously, with a duration of said intervals being sufficient toregenerate said NO_(x) trap.
 8. An engine according to claim 7, whereinsaid intake system comprises a compensation means to reduce air supplyto each said bank of cylinders when each said bank is activatedsimultaneously, thereby avoiding a sudden change in engine output.
 9. Anengine according to claim 8, wherein said compensation means comprisesan electronically actuated throttle coupled to an electronic controlsystem.
 10. An engine according to claim 8, wherein said compensationmeans comprises an ON/OFF valve arranged in series with a bypassthrottle arranged in a bypass passage bypassing a main throttle arrangedin a main passage, with said main and bypass throttles being ganged suchthat flow through said main and bypass passages when said ON/OFF valveis open is in a same predetermined ratio to a mass air flow through saidmain throttle alone.
 11. An engine according to claim 7, wherein, whenone bank of cylinders is deactivated, said one bank of cylinderscommunicates with ambient air through a diverter valve so that airsupplied to said one bank of cylinders is unthrottled.
 12. An engineaccording to claim 11, wherein said diverter valve comprises:two inletsconnected to ambient air and said main throttle, respectively; twooutlets, each connected to an intake manifold of a respective bank ofcylinders; and, a rotatable element having a first position, in whichsaid inlet connected to ambient air is obstructed while said inletconnected to said main throttle is connected simultaneously to bothoutlets, and a second position, in which said main throttle is connectedto only one of said outlets while said inlet connected to ambient air isconnected to the other of said outlets.